This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
The present disclosure relates generally to medical devices and, more particularly, to sensors used for sensing physiological parameters of a patient. In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such physiological characteristics. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.
One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry may be used to measure various blood flow characteristics, such as the blood-oxygen saturation of hemoglobin (SpO2) in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time varying amount of arterial blood in the tissue during each cardiac cycle.
Pulse oximeters typically utilize a non-invasive sensor that transmits light through a patient's tissue and that photoelectrically detects the absorption and/or scattering of the transmitted light in such tissue. One or more of the above physiological characteristics may then be calculated based upon the amount of light absorbed and/or scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed and/or scattered by the blood in an amount correlative to the amount of the blood constituent present in the blood. The amount of light absorbed and/or scattered may then be used to estimate the amount of blood constituent in the tissue using various algorithms. This determination may be performed in a monitor coupled to the sensor that receives the necessary data for the blood constituent calculation.
Conventional two wavelength pulse oximeters emit light from two light emitting diodes (LEDs) into a pulsatile tissue bed and collect the transmitted light with a photodiode positioned on an opposite surface (transmission pulse oximetry) or on an adjacent surface (reflectance pulse oximetry). The LEDs and photodetector are housed in a reusable or disposable sensor which communicates with the pulse oximeter. For estimating oxygen saturation, at least one of the two LEDs' primary wavelengths is typically chosen at some point in the electromagnetic spectrum where the absorption of oxyhemoglobin (HbO2) differs from the absorption of reduced hemoglobin (Hb). The second of the two LEDs' wavelength is typically at a different point in the spectrum where, additionally, the absorption differences between Hb and HbO2 are different from those at the first wavelength.
The first LED is typically configured to emit light with a wavelength in the near red portion of the visible spectrum 660 nanometers (nm) and the second LED is configured to emit light with a wavelength in the near infrared portion of the spectrum near 900 nm. The near 660 nm-900 nm wavelength pair has been selected because it provides for the best accuracy when SpO2 is high (e.g., in the 85% and above range). Some pulse oximeters replace the near 660 nm LED with an LED configured to emit light in the far red portion of the spectrum near 730 nm. The near 730 nm-900 nm wavelength pair has been selected because it provides for the best accuracy when SpO2 is low (e.g., in the range below 75%). Unfortunately, inaccuracies result from using a single wavelength pair. The single pair of wavelengths can only properly account for a portion of the entire arterial oxygen saturation range.